In recent years, for various applications, an optical filter which sufficiently transmits light in the visible wavelength region and which blocks light in the near ultraviolet wavelength region and the near infrared wavelength region has been used.
Hereinafter, “visible wavelength region”, “(near) ultraviolet wavelength region” and “(near) infrared wavelength region” will be referred to as “visible region”, “(near) ultraviolet region” and “(near) infrared region”, respectively, and “light in the visible wavelength region”, “light in the (near) ultraviolet wavelength region” and “light in the (near) infrared wavelength region” will be referred to as “visible light”, “(near) ultraviolet light” and “(near) infrared light”, respectively.
For example, in imaging devices using a solid-state imaging element (e.g. CCD or CMOS), such as a digital still camera, or in display devices using a light receiving element such as an automatic exposure meter, in order to achieve favorable color reproducibility, an optical filter has been used. The spectral sensitivity of the solid-state imaging element or the light receiving element ranges from the ultraviolet region to the near infrared region, whereas human eyes can detect only visible light. Accordingly, in order to make the spectral sensitivity of the solid-state imaging element or the light receiving element close to the luminous factor of a human being, an optical filter is disposed on the subject side of the solid-state imaging element.
For such an optical filter, various systems are employed, for example, a reflection type filter in which dielectric thin films differing in the refractive index are alternately stacked (dielectric multilayer film) on one side or both sides of a transparent substrate to reflect light to be blocked employing interference of light. Of a filter having a dielectric multilayer film, optical properties may change since the optical film thickness of the dielectric multilayer film changes depending upon the angle of incidence of light. Accordingly, if such a filter is used, the spectral sensitivity of the solid-state imaging element may be influenced by the angle of incidence.
Whereas, Patent Documents 1 and 2 disclose as an optical filter which is less influenced by the angle of incidence of light at a wavelength of from 600 to 800 nm, one absorption type filter having an absorbing layer containing an absorbing dye in a transparent resin, or a filter having a dielectric multilayer film and an absorbing layer in combination. Further, the present applicant proposes an optical filter with reduced dependence on the angle of incidence of optical properties for light having a wavelength of from 600 to 800 nm (transmittance wavelength dependence) by incorporating a squarylium compound having a specific structure into a transparent resin (Patent Document 3). As mentioned above, of a filter having an absorbing layer, a change of optical properties by the angle of incidence of light tends to be small, whereby the influence of the angle of incidence of light having a wavelength of from 600 to 800 nm over the spectral sensitivity of the solid-state imaging element can be reduced.
Further, Patent Documents 4 to 6 disclose an optical filter having an absorbing layer containing a compound which absorbs light having a wavelength of from 380 to 420 nm. These Patent Documents disclose that the dependence on the angle of incidence of light having a wavelength of from 380 to 420 nm can be reduced.
Further, along with an improvement of a solid-state imaging element, an optical filter is required to be such that the wavelength at which the transmittance is 50% is at least 400 nm and that a change between a wavelength at which the transmittance is about 15% and a wavelength at which the transmittance is about 70% is steep.
Patent Documents 3 to 5 discloses, so as to reduce the dependence on the angle of incidence in the near ultraviolet region, an azomethine compound, an indole compound, a benzotriazole compound and a triazine compound.
However, with such compounds, the optical filter does not have a sufficient near ultraviolet absorbing performance and does not have a maximum absorption wavelength in an appropriate wavelength band. Accordingly, in order that the wavelength at which the transmittance is 50% is at least 400 nm efficiently, it is necessary to increase the amount of addition of such a compound or to increase the film thickness of the transparent resin.
However, if the amount of addition of the compound is increased or the thickness of the transparent resin is increased so as to satisfy the above optical properties, properties of the resin may be impaired. For example, in a case where a dielectric multilayer film as a light reflecting layer is formed on a glass substrate by deposition, a stress may occur between layers by the heating/cooling procedure in the evaporation step, and cracking or breakage may occur, and the possibility is increased also by a heating step in the production process other than the evaporation step. Further, in a case where an absorbing layer is formed on a glass substrate or a film substrate, the adhesion of the absorbing layer to the substrate tends to be inferior and separation may occur, and such is problematic in the reliability.
In order to solve the above problems and to secure sufficient shielding properties of the absorbing layer against light in both the near infrared region and the near ultraviolet region with a small amount of addition of the compound, both the near infrared absorbing dye and the near ultraviolet absorbing dye used are required to have a high absorptivity and have a sharp spectral transmittance curve (in this specification, the spectral transmittance curve will sometimes be referred to as “transmission spectrum”) in a predetermined wavelength band.
However, an azomethine compound, triazine compound or benzotriazole compound dye has problems such that no sharp transmission spectrum in a predetermined wavelength band will be obtained, and such a compound is inferior in the heat resistance, and a change in optical properties is likely to occur in the thermal process.